This invention relates generally to the field of optical microscopy, and more particularly to near-field scanning optical microscopy for high-resolution imaging.
Microscopes employing conventional optical imaging systems cannot resolve features substantially smaller than about one-half an optical wavelength, because of diffractive effects. However, near-field scanning optical microscopy (NSOM) can be employed to achieve finer resolution in optical imaging. In NSOM, an aperture having a diameter that is smaller than an optical wavelength is positioned in close proximity (i.e., within less than one wavelength) to the surface of a specimen and scanned over the surface. Light may be either emitted or collected by such an aperture. The aperture is defined in the end of a probe. Mechanical or piezoelectric means are provided for moving the probe relative to the sample. Light that has interacted with the sample is collected and detected by, e.g., a photomultiplier tube. The strength of the detected light signal is typically stored, in the form of digital data, as a function of the probe position relative to the sample. The stored data can be displayed on, e.g., a cathode-ray tube as an image of the scanned surface.
One approach to the design of NSOM probes has been described in U.S. Pat. No. 4,917,462, issued to A. Lewis, et al. on Apr. 17, 1990. According to that approach, the probe is a highly tapered, glass pipette. The optical aperture is defined at the narrow end of the pipette, where the capillary bore forms an orifice. The outer surface of the pipette is coated with metal, typically aluminum, in order to increase the opacity of the glass wall. The aperture is defined by metallizing the annular region at the very end of the pipette, surrounding the orifice. The resulting tip aperture is readily made less than 1000 .ANG. in diameter, or even smaller.
The pipette behaves approximately like a classical metallic waveguide. Light is transmitted through the pipette in a propagating mode or combination of modes. A cutoff diameter is associated with each such mode. Generally, the outer diameter of the glass wall of the pipette is everywhere greater than the cutoff diameter of the lowest desired mode. The cutoff threshold of the lowest mode is reached only at the thin metallized region at the tip of the pipette.
A second, aperture-based approach to NSOM probes has been described in U.S. patent application Ser. No. 615,537, filed on Nov. 19, 1990. According to that approach, the probe is made from a tapered, single-mode optical fiber having a flat end portion. At least a terminal portion of the tapered fiber is coated with metal on the outer walls. Optionally, the end flat is also overcoated with metal. The metal layer overlying the end flat is formed as an annulus, the bare portion within the annulus defining the optical aperture of the probe. Analogously to the pipette probe, the metallized terminal portion of the fiber probe behaves like a metallic waveguide. The diameter of the fiber falls to the cutoff diameter of the lowest mode of interest at or near the end flat.
Although useful, the aperture-based probes described above suffer several disadvantages.
For example, it has been noted that there are cutoff diameters associated with the metallic waveguiding properties of the probes. If apertures are made substantially smaller than the cutoff diameters for the modes of interest, the attainable signal is substantially decreased. As a consequence, there are practical limits on how small the tip diameter can be made while still providing a useful signal. Furthermore, the thickness of the metal layer contributes to the overall diameter of the probe tip. Thus, both the tradeoff between aperture size and signal strength, and the thickness of the metal layer, impose limits on the smallest practical tip size.
Signal transmission through small apertures is fraught with other difficulties in addition to the problem of attenuation in a below-cutoff waveguide. For example, the maximum signal that can be collected by an idealized aperture of diameter D in an infinitesimally thin screen is theoretically proportional to D.sup.6. Thus, it is clear that the signal drops rapidly in strength as the aperture is reduced in size.
Yet another disadvantage is that the ultimate achievable resolution is limited by the finite electrical conductivity of any metal coating. That is, if the metal coating were a perfect conductor, the guided electromagnetic field would not penetrate into the metal. However, because any actual metal used for coating the fiber has some electrical resistivity, and therefore a finite conductivity, it is inevitable that the electromagnetic field will extend some distance into the metal. This is true, in particular, at the aperture. As a consequence, the effective aperture (for purposes of image resolution) is somewhat larger than the metal-free region at the end of the fiber. Instead, the effective aperture extends into the surrounding metallized region. Because of this, it is difficult, as a practical matter, to resolve features substantially smaller than about 100 .ANG. in extent, even with the finest aperture-based probes.
Practitioners in the field have hitherto been unable to circumvent the limitations discussed above. As one consequence, efforts to make extremely high-resolution probes, e.g., probes capable of resolving features smaller than about 100 .ANG. while providing signal-to-noise ratios large enough for practical imaging, have been frustrated.